1. Field of the Invention
This invention relates to a composite concentration system, and more particularly a concentration system for concentrating subscriber lines of a digital local switching system.
2. Description of the Prior Art
As is well known in the art, the size and scale of a local switching system are determined by taking into consideration such factors as the number of subscriber lines, the content of the service to be provided (percentage of call falls, for example), the types of subscribers and the type and scale of the system.
The local switching systems generally used are classified into two types, one a space division system, and the other, a time division system.
In a space division system the subscriber lines connected to telephone terminal sets of individual subscribers are firstly accommodated in a space division concentration stage. In this stage, the lines are collected at different concentration ratios for the different types of subscribers, such as residential subscribers or lines as well as for public uses. For example, since the frequency use of common residential subscriber lines is generally low, the line concentration ratio is 10:1, whereas in the public subscriber lines, where frequency of utilization is high, the lines are concentrated at a concentration ratio of 2:1. In this manner, the subscriber lines are concentrated into fewer signal lines than the quantity of subscriber lines. The signals of these signal lines are multiplexed through A-D and D-A converters constituting the subscriber line interface circuit and then are accommodated in a distribution stage.
In a space division system, as described above, the subscriber lines are concentrated at the space division concentration stage and then are connected to the subscriber line interface circuits so that, when considered in the units of the subscribers, the number of expensive subscriber line interface circuits can be reduced, thus decreasing the installation cost of the whole system. The space division concentration circuits utilized in this stage, however, must have a different construction for different types of subscribers and different types of subscriber line interface circuits. This not only complicates the overall system but also increases the cost. Moreover, since the controls of the space division concentration stage should be different depending upon the types of the subscribers and subscriber line interface circuits, it is difficult to adequately and economically improve the service performance by adding a new subscriber line interface circuit in response to an increase in the quantity of subscriber lines, and an increase in the number of calls due to increases in the frequency of use.
On the other hand, in a time division system, each subscriber line is accommodated in a time division concentration stage through A-D and D-A converters consisting of a single subscriber line interface circuit. At the concentration stage, the traffic in which calls and pagings for m subscriber lines are made is limited to n channels by a time switch, and the signals are then accommodated in a distribution stage through a digital highway. In a time division system, since it is necessary to provide a subscriber line interface circuit for each subscriber line, and since it is necessary to provide a buffer memory device for each subscriber line in order to execute a time switch processing, a time division system is more expensive than a space division system at a low percentage of calls. It is not necessary, however, to provide a space division concentration stage consisting of various combinations of circuit elements at the preceding stage of the subscriber line interface circuit. Accordingly, the construction of the time division system is definite or fixed and simple, irrespective of high or lower percentage of the calls. For this reason, a time division or concentration system becomes less expensive than a space division system at high percentage of calls. Since the time division has a simple construction, addition of the service and new capabilities to the subscriber line interface circuit can be made more easily than the space division system without increasing the installation cost.
As above described, the space division system and the time division system have inherent advantages and disadvantages so that, when installing a telephone exchange in a district, the system to be adapted and the construction of the subscriber lines are determined by considering the type of subscriber, the number of the subscriber lines necessary in that district, the service required, and the percentage of calls.
Generally, however, the number of the subscribers, the type thereof, and service vary with time so that it is necessary to change the concentration system of the subscriber lines in accordance with such variation. In the prior art, although the subscriber lines have been designed by forecasting the future increase in the number of subscriber lines, their design is fixed once the telephone system is installed and, therefore, it is not possible to modify the system in the future. Accordingly, when changing the concentration system of the existing subscriber lines it is necessary not only to exchange substrates but also to change wirings resulting from the exchange of the substrates. Such operations require much labor and time and also must be tested after modification.